Platform motion as ch goes 2 to 20 degrees

apr07

Introduction:

All previous pointing models have been made with the
carriage house (ch) at stow position (za=8.84 degrees). When the dome
is at a large za and passes in front of a tower the tie down cables go
slack causing a problem with the pointing. If we moved the rest
position of the carriage house up to say 18 degrees za then the tie
down
cables would not go slack as quickly. For this to work we need to check:

How much "temperature range" will we gain by moving the ch
up to 18 degrees. How much will this lift the dome side.

The path of the dome will now follow a different track in focus.
Will this make the current focus error better or worse?

The azimuth rails were shimmed with the ch at 8.84 degrees. Will
moving out to 18 degrees affect things like the 3 azimuth term?

The setup:

The dome was set to 19.6 degrees. The azimuth was at 362.87 (dome
side) pointing at tower 12.

The tie downs were turned off and the distomats were run on a 1
minute update rate.

The tilt sensors in the dome were also sampled at 5 Hz.

The carriage house was driven from za=2 to za=19.5 at 1
minute/deg. This gave 1 distomat sample per degree.

There were two complete cycles of the ch going up/down.

The computations and plots:

The average height of the platform was subtracted from each
distomat sample. This removed the temperature dependence of the motion.

Page1 top Vertical deflection
of Corner 12 vs ch za: This shows the vertical deflection of
corner 12 as the ch moved from 2 to 20 degrees and back two times. A
linear fit gave the deflection as: CornerDeflInches=-6.82313 + .07560
*zaCh. Negative direction is toward the dish.

Page1 Center Tilt sensor (in
dome) pitch vs ch za. A linear fit to the data was: PitchDeg=
9.77005 + .00426 * chZa. The constant term has not been calibrated to
the actual tilt of the dome relative to the dish. The important value
is the slope of the line.

Page1 Bottom Plotting platform
tilt and dome tilt vs chza. The corner deflection was changed to
an angle using the distance 124.7 feet (center triangle to corner). The
platform tilt slope is less than the dome slope as the chZa changes:
.003 to .004 for dome.

Lynn baker pointed out that this might be the change in the
deflections in the azimuth rails as the ch goes to higher za. As the ch
moves up , the az rails on the dome side should deflect less causing
the dome side to be higher (increasing the pitch). This would not
affect the tilt of the platform.

When azimuth spins were done with the tilt sensors at
za's from 2 to 19 degrees the amplitude of the 3 azimuth term did not
increase as the dome moved to higher za (see az
swings page 5 where pitch 3 az amplitude is plotted vs dome
za). If what lynn said was true, you would have expected the 3 az
term to increase as the dome za increased since the deflection in the
az rail should also change.

Page 2 Dome pointing error and
Focus change vs ch za.

The top plot shows the change in the pointing error vs ch
za. The error is relative to the ch sitting at stow (8.84 degrees
.. the dashed line). Moving the ch from 8.8 to 18 degrees (3rd line
from the top) changes the pointing error by 15 to 20 arc seconds on the
sky.

The bottom plot shows the dome focus error for the zach 2 to 20
degrees. Moving the ch from 8.8 to 18 degrees za changes the
focus position from .1 to .8inches. The positive direction means that
the focal length is longer (the dome has moved away from the dish as
the ch moved up in za.

Page 3 The focus error with ch
az 8.8 and 18 degrees za. The focus error was measured with the
theodolite back in feb03. Focus curves were then done in feb03 and
apr/may06.

Focus
curves taken with sbn (.ps) (more
info) show the offset (in inches) from the the platform reference
position (1256.22 feet). The carriage house was at 8.84 za when this
data was taken. When the number is positive, the platform must be above
the reference to focus. Between za=5 and za=15 most of the sources are
focusing below the reference position. This implies that the focal
length is too long at these zas. Outside this range the the focal
length is too short.

Moving the ch from 8.84 to 18 degrees will:

increase the path length .3 inches when the dome is at za=10.
This will make the focus worse since it is already too high by about .5
inches.

increase the path length by .8 inches when at za=20. This
will improve the focus since it is probably .6 inches too low at za=20.

Conclusions:

When the dome points at a corner:

Moving the ch in za causes the platform corner to move by
.0756 inches per degch za.

Moving the ref ch position from 8.84 to 18 degrees will raise the
platform corner by .7 inches.

.7 platform inches is 1.7*.7 is 1.2 tie down inches of stroke

.7 platform inches is 3.5 degrees F.

The path length will increase by .1 in (za=2) to .8 inches
(za=20).

Looking at the focus curves, the increased path length will
make the focus worse between za=5 and za=15. At za = 10 the focus error
will go from .5 inches too high to .8 inches too high.

We probably don't want to move the ch to za=18 because of the
focus for the high frequency receivers.